Currently, the 3rd Generation Partnership Project (3GPP) is developing Long Term Evolution (LTE), also referred to as E-UTRAN, as set out in the technical specification 3GPP TS 36.300 v 8.5.0 (2008-05), to which the reader is referred for additional information, and related documents. 3GPP LTE aims to enhance the Universal Mobile Telecommunications System (UMTS) Radio Access Network standard, for example, by improving efficiency and services.
In LTE, user equipment (UE) communicates with a network node, E-UTRAN NodeB (eNB), with data being sent on radio bearers (RBs) over a radio link between them. The eNB interfaces with a Mobile Management Entity (MIME) via an interface designated as S1. An LTE network typically includes a plurality of eNBs and MMEs as illustrated schematically in FIG. 1 (S1-flex mechanisms allow an eNB to be connected to a plurality of MMEs).
In a typical UMTS network, a plurality of Radio Access Controllers (RNCs) each control a plurality of base stations B as illustrated schematically in FIG. 2. Mobile devices UE connect to the network via the base stations.
Where a mobile device (UE) is connected to an LTE network, it may be necessary to handover its connection to another network, for example, if signal conditions change or if the mobile device is moving, or for some other purpose. Thus, a UE might be transferred from a cell in an LTE network to a UMTS network having a neighbouring cell or cells.
As part of the LTE-UMTS automatic neighbour relation function (ANRF), the eNB to which the UE is attached may be informed by the UE that it has detected a new neighbour UMTS cell. The source eNB may request the UE to obtain, from the information broadcast by the UMTS RNC to which that cell belongs, the global cell id of that neighbour cell, which uniquely identifies that cell within the UMTS network (see 3GPP TS 36.300 v 8.5.0 section 22.3.4), and might also request the Location Area Identification (LAI) and the Routing Area Code (RAC) broadcast in the neighbour cell. The global cell id is also referred to as the UC-id or UTRAN global cell id. The UTRAN cell id UC-id includes the RNC identifier (RNC-id) and the cell (C-id), where UC-id=RNC-id+C-id (see 3GPP TS 25.401 v 8.0.0 section 6.1.5).
The UE is not always able to read the RNC-id (RNC identifier) of the RNC to which that cell belongs. There may be circumstances where the RNC-id cannot be obtained by applying a mask to the global UTRAN cell id/UC-id. Although the UC-id includes the RNC-id and the cell C-id (UC-id=RNC-id+C-id), the length of this can vary. In most networks UC-id (28 bits)=RNC-id (12 bits)=+C-id (16 bits). However, typically for flat areas there may also be UC-id (28 bits)=RNC-id (16 bits)+C-id (12 bits). Thus it may not be possible to detect the RNC-id using masking. As the RNC-id length of the discovered neighbouring cell is unknown, handover towards this cell from the LTE source eNB cannot be carried out because RNC-id is required for the routing of the handover message to instigate handover. In 3GPP TS 36.413 (S1-AP specification) the target cell ID for LTE-to-UMTS handover is included in the “HO Required” message sent by the source eNB and defined in section 9.2.1.6 as the target RNC-id.
As an alternative to ANRF, eNBs of an LTE network may be configured by Operation and Maintenance (O&M), when the LTE network is initially set up, to include information about the surrounding UMTS RACs where applicable, and to include a list of the RNC-ids that comprise these RACs. This requires significant effort in respect of a large number of eNBs to obtain comprehensive information about the configuration. If a change in the UMTS network occurs, for example, if a new base station is added or RAC allocation is changed, the neighbouring nodes of the LTE network need to be updated in the field.